1. Field of the Invention
The present invention generally relates to an access network device and a management method thereof. Particularly, the present invention relates to an access network device for managing queue by determining feasibility of real-time transmission within a predetermined time in advance and dropping a packet determined to be infeasible to transmit, and a method thereof.
2. Description of the Related Art
Internet is prevalent all over the world thanks to developments of computer and communication technologies. Much research has been made on wireless Internet systems for active uses of the Internet and computers. The wireless Internet is briefly classified into a wireless stationary Internet and a wireless mobile Internet. The wireless stationary Internet includes systems for wireless LAN (Local Area Network), B-WLL (Broadband Wireless Local Loop), LMDS (Local Multipoint Distribution Service), and Bluetooth. The wireless stationary Internet has limits in mobility, but provides superior transmission capacity and speed. By contrast, the wireless mobile Internet, which is implemented by different types of systems in a notebook computer and PC according to generations of the mobile communication network, provides superior mobility, but has limitations in transmission capacity and speed.
In order to support mobility in such a mobile network, a MIP (Mobile IP) and a QoS (Quality of Service) that secures a reliable transmission of data and multimedia traffic are required. The reliable transmission of the multimedia traffic in wired network environment uses an RSVP (Resource reSerVation Protocol) which reserves network resources in advance.
However, since the present RSVP is not developed in consideration of the mobility, a proper resource reservation cannot be made when the MIP, which is the mobility protocol, is used. Research for the resource reservation in the mobile network is continuously in progress after the introduction of the MIP. The representative protocols of such research include an MRSVP (Mobile RSVP), an RSVP-MP (RSVP Mobility Proxy), and a CORP (a method of Concatenation and Optimization for resource Reservation Path).
FIG. 1 illustrates a wireless network environment in which a mobile node (MN) 10 is connected to a correspondent node (CN) 20 through an access network and a backbone core network. The MN 10 is generally a notebook computer, a PC, or a mobile phone. The MN 10 accesses the access network through an access router (AR) so as to communicate with other nodes in the access network or with the CN 20 connected through the backbone core network.
The connection through the wireless link of FIG. 1 is superior to that through a wired link in view of the mobility, but has a higher error rate in the packet transmission due to the deterioration of the transmission capability caused by noise, interference and distance.
Basic methods for dealing with errors at a link layer include a forward error correction (FEC) mechanism and an automatic repeat request (ARQ) mechanism. These mechanisms may be separately used to obtain optimum performance, or, used in combination such as a hybrid ARQ.
The FEC mechanism attaches additional data to transmitted packets so that a reception side can recover the losses when there are lost packets. Since the packets are transmitted just one time between a reception terminal and a transmission terminal, the FEC mechanism is suitable for real-time communication. However, since a code for error recovery is additionally required, the additional data prolongs the processing and increases overhead. The FEC mechanism wastes resource bandwidth due to the necessity of transmitting the additional data even when a link status is stable and normal.
The ARQ mechanism determines the loss of packets and retransmits the packets if an acknowledgement (ACK) packet is not received within a predetermined time-out with respect to each packet transmitted from the transmission side. A router, which interconnects networks in Internet protocol (IP)-based wireless network environment, transmits packets to a mobile node and then, retransmits the packets if the ACK packet is not received from the mobile node.
Currently, communications among users are performed more rapidly and the users experience real-time TV broadcasts and videoconferences through PCs. When such real-time communication is required, transmission of real-time packets from the reception side to its destination side has restrictions on time, and thus the packets transmitted to the destination are dropped after the deadline. Specifically, real-time traffic for transmitting packets in real-time receives the packets in real time and utilizes the received packets. Thus, packets beyond a predetermined time after the transmission are dropped and the following packets are utilized.
In the wireless access environment as shown in FIG. 1, the mobile node needs to connect with the AR to access the access network. The AR, which is connected with a certain number of upper routers, forwards packets from the CN 20 to the mobile node 10, or, forwards packets from the mobile node 10 to the CN 20 along a reserved route.
The AR having a transmission queue buffers the received packets in the transmission queue and forwards the received packets to the destination by checking an IP address of the destination contained in a header of the received packets. The queue represents a first-in first-out (FIFO) buffer structure which outputs in a sequential order data stored in a constrained memory area. The transmission queue indicates the queue storing the packet to be transmitted. When the packets to be transmitted are stored in the queue, the packets to be transmitted await in the queue until the firstly stored packets are output.
Packets for real-time transmission are stored in the transmission queue and then, transmitted in a sequential order of their storage. Accordingly, queuing time becomes quite long until the requested packets are transmitted because all prior transmission packets are transmitted first.
If there is a plurality of prior transmission packets and a considerable queuing time is required for the router in the network, total time for transmitting the packets to the destination increases. Consequently, the deadline may expire before the packets arrive at the destination. Such real-time transmission packets beyond the deadline are invalid and thus, the destination drops the packets upon the reception.
In the above conventional arrangement, the packets, which are dropped due to the arrival beyond the deadline in the real-time communication, need to arrive at the destination. Hence, the AR stores the packets in the transmission queue and forwards the packets to the destination, thus wasting resources such as the bandwidth.
This disadvantage becomes worse in the network environment implemented under the ARQ mechanism. For the real-time communication under the ARQ mechanism, the router has the transmission queue for storing the packets to be transmitted to the mobile node at the link layer and also has a retransmission queue for storing a duplicate of the packets to be transmitted. When the ACK packet is received from the mobile node, the related duplicate packet stored in the retransmission queue is removed at once. Packets that are negatively acknowledged (NACK) are moved from the retransmission queue to the transmission queue and are transmitted prior to other packets pre-stored in the transmission queue. Therefore, the retransmission due to such errors requires more time and increases probability to exceed the deadline of the real-time traffic. The resources of the retransmission queue are wasted since the router has both of the transmission queue for the packets and the retransmission queue for the duplicates of the packets.
Efficient queue management methods have not been suggested for dealing with the above disadvantages in the wireless network. Resource management methods based on the queue management suggested for the wired network, such as RED (Random Early Detection), SRED (Stabilized RED), BLUE, and GRED (Gentle RED), do not consider characteristics of the wireless network and are not applicable.
In case of frequency handover in the wireless network environment, both an old AR (OAR) and a new AR (NAR) store the packets, thus wasting resources.